Naltrexone is a pure opiate antagonist with no observable agonist effects in humans. Opioid antagonists bind to the opiate receptor site and block the pharmacological effects of opioid agonists, such as heroin and morphine. For this reason, naltrexone and other opioid antagonists have been investigated as potential therapeutic agents in the treatment of opioid dependence. This research will examine an investigational new depot formulation of naltrexone. One advantage of a depot formulation is that a single dosing might improve patient compliance and retention in treatment, compared to the currently available oral form of naltrexone (Trexan) that requires daily or three times per week dosing. This research project will be conducted in two studies. The first study will assess the safety and acceptability of an subcutaneous injection of depot naltrexone in terms of untoward side effects and local skin irritation using normal volunteers tested in an outpatient setting. The second study will be conducted after completion of the first study and will assess the degree and duration of opioid blockade of depot naltrexone in current opiate users. Subject testing for the first study is expected to begin in August, 1992.